This invention relates to an improved process for the conversion of aromatic hydrocarbons. More specifically, the invention concerns disproportionation and transalkylation of aromatic hydrocarbons to obtain xylenes through the use of a zeolitic catalyst.
The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is paraxylene, the principal feedstock for polyester which continues to enjoy a high growth rate from large base demand. Orthoxylene is used to produce phthalic anhydride, which has high-volume but mature markets. Metaxylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Most commonly, toluene is dealkylated to produce benzene or disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered. More recently, processes have been introduced to disproportionate toluene selectively to obtain higher-than-equilibrium yields of paraxylene.
A current objective of many aromatics complexes is to increase the yield of xylenes and to de-emphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. Benzene produced from disproportionation processes often is not sufficiently pure to be competitive in the market. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C9 aromatics along with toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,016,219 (Kaeding) discloses a process for toluene disproportionation using a catalyst comprising a zeolite which has been modified by the addition of phosphorus in an amount of at least 0.5 mass-%. The crystals of the zeolite are contacted with a phosphorus compound to effect reaction of the zeolite and phosphorus compound. The modified zeolite then may be incorporated into indicated matrix materials.
U.S. Pat. No. 4,097,543 (Haag et al.) teaches toluene disproportionation for the selective production of paraxylene using a zeolite which has undergone controlled precoking. The zeolite may be ion-exchanged with a variety of elements from Group IB to VIII, and composited with a variety of clays and other porous matrix materials.
U.S. Pat. No. 4,629,717 (Chao) discloses a phosphorus-modified alumina hydrogel formed by gelation of a homogeneous hydrosol. The composite has a relatively high surface area of 140-450 m2/g and high activity and selectivity in 1-heptene conversion tests.
U.S. Pat. No. 4,724,066 (Kirker et al.) teaches a hydrocarbon dewaxing process using a catalyst comprising a zeolite and a crystalline aluminum phosphate; aside from the differences in process, Kirker differs from the present invention in specifying a crystalline rather than amorphous aluminum phosphate component.
U.S. Pat. No. 5,169,812 (Kocal et al.) teaches a catalyst for aromatization of light hydrocarbons comprising a zeolite, preferably ZSM-5, a gallium component and an aluminum phosphate binder. The composite is treated with a weakly acidic solution, dried and calcined to increase its tolerance to hydrogen at high temperatures.
U.S. Pat. No. 4,011,276 (Chu) presents a process for the disproportionation of toluene using a catalyst comprising a crystalline aluminosilicate zeolite such as MFI which has been modified by the addition of oxides of phosphorous and magnesium.
U.S. Pat. No. 4,182,923 (Chu) teaches a high conversion process to disproportionate toluene to benzene and xylenes rich in para-xylene. The process employs a crystalline aluminosilicate zeolite such as MFI which has been modified by treatment with ammonium hydrogen phosphate to deposit at least 0.5 weight percent phosphorous.
Workers in the field of aromatics disproportionation continue to seek processes and catalysts having exceptionally high selectivity for paraxylene from toluene combined with favorable activity and stability.
It is an object of the present invention to provide an improved process for the disproportionation of aromatic hydrocarbons to yield desirable alkylaromatic isomers. A specific objective is to obtain a high yield of paraxylene by disproportionation of toluene.
This invention is based on the discovery that high activity with potential for selectivity to paraxylene is obtained by disproportionation of toluene using a zeolitic catalyst which has been oil-dropped with an amorphous aluminum phosphate binder.
The present invention therefore is directed to a process for the disproportionation of a toluene feedstock to obtain a product comprising paraxylene using an oil-dropped spherical catalyst comprising a zeolitic aluminosilicate having a pore diameter of from about 5 to 8 xc3x85 and an amorphous aluminum phosphate binder. The catalyst optionally has an enhanced surface silicon content. Preferably the product contains paraxylene in excess of its equilibrium concentration at disproportionation conditions. The preferred catalyst of the present invention comprises a zeolitic aluminosilicate preferably selected from MFI, MEL and MTW, and most preferably comprises MFI. In one embodiment, the catalyst has a particle size of no more than about 1 mm.
The catalyst preferably is subjected to a precoking step prior to its use for disproportionation/transalkylation in order to deposit a controlled concentration of carbon on the catalyst and increase its selectivity to paraxylene in the product.
A process combination optionally comprises a xylene-separation zone; preferably, paraxylene is recovered by adsorption or a combination of adsorption and crystallization.
These as well as other objects and embodiments will become apparent from the detailed description of the invention.